The present invention relates generally to gas sensors and, more particularly, to carbon monoxide sensors.
Carbon monoxide sensors are used in a wide variety of applications, including the monitoring of heating installations that employ fossil fuels as an energy source and the monitoring of exhaust fumes from internal combustion engines. Two additional applications involve self-cleaning ovens and fuel cells.
Specifically, self-cleaning ovens include a cleaning cycle that removes carbonaceous residues through a high-temperature burning at a high-power capacity for a fixed amount of time. Because the high-temperature burning consumes a relatively large quantity of energy, there is a need for efficient self-cleaning cycles that automatically shut off the oven as soon as the burning process is complete. One way to accomplish this would be to monitor the carbon monoxide evolution during the heating cycle. Specifically, it is known that a typical dirty oven, when being cleaned at temperatures exceeding 800xc2x0 F., will begin to emit carbon monoxide at a temperature of about 550xc2x0 F. The amount of carbon monoxide being emitted will peak at around 800xc2x0 F. at a value of about 1500 ppm. After this peak value is reached, the carbon monoxide concentration decreases to around 200 ppm at the end of the cycle. An appropriate shut down point will occur for a typical oven at a carbon monoxide concentration of about 200 ppm.
Therefore, there is a need for a robust, high-sensitivity carbon monoxide sensor to monitor self-cleaning oven cycles and other carbon monoxide emitting devices. Further, the sensor must be sensitive so as to detect a relatively low concentration of carbon monoxide of less than 50 ppm.
Currently-available carbon monoxide sensors include infrared adsorption sensors and thin film metal oxide technology, such as tin oxide sensors. The infrared adsorption sensors are inappropriate for the household oven market due to their high cost and low sensitivity. The thin film metal oxide sensors are also inappropriate for use in monitoring self-cleaning oven cycles because they generally don""t work well in a humid environment. Further, metal oxide sensors take a long time to regenerate.
Accordingly, there is a need for a low-cost, fast-response, and high-sensitivity carbon monoxide sensor for use in self-cleaning ovens and other carbon monoxide emitting devices.
As noted above, another application for carbon monoxide sensors is in connection with fuel cells. Fuel cells are known devices that convert chemical energy of a fuel to electrical energy. Each fuel cell includes a pair of electrodes arranged across an electrolyte. The surface of one electrode is exposed to hydrogen or a hydrogen-containing gaseous fuel and the surface of the other electrode is exposed to an oxygen-containing oxidizing gas. Electrical energy is produced at the electrodes through electrochemical reactions. Typically, an adsorbent is used on the surface of the anode that is exposed to hydrogen or the hydrogen-containing gaseous fuel. One known problem associated with fuel cells is the poisoning of this adsorbent by the adsorption of carbon monoxide.
Therefore, there is also a need for a robust, high-sensitivity carbon monoxide sensor for monitoring the carbon monoxide level of the hydrogen or hydrogen-containing gaseous fuel fed to the anodes of a fuel cell. Again, existing infrared sensors are undesirable due to their high cost and low sensitivity and thin-film metal oxide sensors are disadvantageous due to their slow regeneration time. Thus, there is a need for a robust, high-sensitivity, and economical carbon monoxide sensor for monitoring the concentration of carbon monoxide in the hydrogen-containing fuel fed to the anodes of a fuel cell.
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
One embodiment of the present invention satisfies the aforenoted need by providing a carbon monoxide sensor that includes a sensing element, a temperature sensor for measuring the temperature of the sensing element, and a signal processing module coupled to the temperature sensor. The sensing element, includes an adsorbent dispersed over a support material. The adsorbent is capable of exothermically adsorbing carbon monoxide.
In other embodiments, the sensor also includes a heating element. Specifically, the heating element heats the sensing element to a temperature that is at least as high as the desorption temperature of the adsorbed carbon monoxide, resulting in a desorption of the adsorbed carbon monoxide and a regeneration of the adsorbent. The preferred adsorbents are monovalent silver (Ag+), monovalent copper (Cu+), and mixtures thereof. The support material may be zeolite, alumina, silica gel, carbonaceous materials, or mixtures thereof. The sensor may include a permeable membrane support substrate disposed on a front side of the sensing element or upstream of the heating element, temperature sensor, and heating element. The sensor may include a protective membrane disposed behind the sensing element, temperature sensor, and heating element.
In still other embodiments, a reference element includes a non-adsorbing material that does not adsorb carbon monoxide. A second temperature sensor is in contact with the reference element. This second temperature sensor is coupled to the processing module and sends signals indicative of the temperature of the reference element to the processing module. A heating element may also be provided for the reference element, or a single heating element may be employed for both the sensing element and reference element.
The present invention also provides a method of detecting a presence of carbon monoxide in a gas. The method includes exposing a sensing element to a gas that may or may not include carbon monoxide. The sensing element includes an adsorbent dispersed over a layer of a support material. The adsorbent is capable of exothermally adsorbing carbon monoxide. The method also includes adsorbing at least a portion of the carbon monoxide in the gas onto the adsorbent, thereby resulting in an increase in the temperature of the sensing element due to the exothermal adsorption. An increase in the temperature of the sensing element can be used as an indication of a presence of carbon monoxide in the gas being tested.
The novel features of the present invention will become apparent to those of skill in the art upon examination of the following detailed description of the invention or can be learned by practice of the present invention. It should be understood, however, that the detailed description of the invention and the specific examples presented, while indicating certain embodiments of the present invention, are provided for illustration purposes only because various changes and modifications within the scope of the invention will become apparent to those of skill in the art from the detailed description of the invention and claims that follow.